Heretofore, a transparent conductive film has been used as a transparent electrode for solar cells or flat panel displays. For these purposes, the transparent conductive film has been typically made of tin-doped iridium oxide (ITO), fluorine-doped tin oxide (FTO) or aluminum-doped zinc oxide (AZO), with particular emphasis on the transparency in the visible region of 400 to 800 nm. As another example, there have also been known non-crystalline transparent conductive films represented by Ga2O3 (Japanese Patent Laid-Open Publication No. 7-335030), Ga2O2—In2O2 (Japanese Patent Laid-Open Publication No. 9-259640), and AB2-xO4-y (A: Mg, Cd, Zn etc., B: Al, In, Ga etc.).
These materials have a bandgap of about 3.2 eV or its equivalent wavelength of about 390 nm, and thereby does not allow transmission of near-ultraviolet light or deep-ultraviolet light having a wavelength shorter than the equivalent wavelength. In the context of no transparent conductive film having a bandgap greater than 5 eV (equivalent wavelength 250 nm) or a transparency in the wavelength range of 240 to 400 nm or 240 to 800 nm, the inventors have precedently developed a transparent conductive thin-film represented by a general formula In2-xYxO3+αwt % of SnO2 or In2-xYxO3+αwt % of Sb2O5, which allows transmission of a light having a wavelength shorter than 400 nm (Japanese Patent Laid-Open Publication No. 2000-90745).
The potential applicability of Ga2O3 or ZnGa2O4 to ultraviolet-transparent conductive materials has been reported up to now. However, there has not been provided any report of verifying transparent conductivity in a thin-film form required for applications to transparent electrodes or antistatic films. The presence of a conductive property in Ga2O3 has been known from a long time ago, such as, in a report of Lowrenz et al. Lowrenz et al. verified 0.03 S/cm of conductivity in a single crystal prepared through a Bernoulli method under reduction atmosphere (Journal Physical Chemistry of Solids, Vol. 28, 1967, p 403).
Recently, Ueda et al. verified 38 S/cm of conductivity in a single crystal prepared through a floating zone method using a rod containing Sn added as a dopant (Applied Physics Letters, Vol. 70, 1997, p 3561). Further, based on a research of anisotropy in conductivity, Ueda et al. verified that Ga2O3 exhibits a higher conductivity in the b-axis direction of the crystal lattice. The b-axis direction means the direction of a line formed of oxygen octahedrons consisting of GaO6, which are connected with each other in a chain structure while sharing their edges. The bandgap in the b-axis direction was 4.79 eV, and a single crystal sample of 0.32 mm thickness exhibited a light transmittance of 20% with respect to a light of 266 nm wavelength. Ueda et al. believe that the Ga2O3 single crystal has a capability of allowing transmission of a KrF excimer laser beam.
The conductive property of Ga2O3 has been methodically Studied by Fleischer et al. Fleischer et al prepared a Ga2O3 thin-film through a sputtering method using a high-purity Ga2O3 target and at a substrate temperature of 500° C. (Thin Solid Films, Vol. 190, 1990, p 93). Fleischer et al. measured 10 kΩ of electric resistance at 1000° C. in the prepared thin-film of 1 μm thickness. Its conductivity was about 0.3 S/cm at 1000° C., but it went down to 0.01 S/cm at a lowered temperature of 800° C. No conductive property has been verified at room temperature.
In Journal of Applied Physics, Vol. 74, 1993, p 300, Fleischer et al. reported that Ga2O3 has a carrier mobility of about 10 cm2/Vs at 1000° C. based on a research of an electric conduction mechanism of Ga2O3 in a temperature range of 800 to 1000° C. However, this report does not mention any conductive property at room temperature.
Recently, Fleischer et al. obtained double-digit increased conductivity by using SnO2 as a dopant (Sensors and Actuators B Vol. 49, 1998, p 110). A thin film was formed through a magnetron sputtering method using a high-purity ceramic target, and then the film was crystallized through a heat treatment at 1050° C. for 10 hours. The thickness of the thin film was from 50 to 200 nm, and the lowest resistance, specifically about 0.5 kΩ at 900° C. and about 100 kΩ at 600° C., was obtained by adding SnO2 at 0.5 mol %.
While this report includes no description of conductivity, this thin film would have a conductivity of about 10 S/cm in view of 1020/cm2 of carrier density at 800° C. in this report and 10 cm2/Vs of carrier mobility in the previous report. However, all of these values are measured in a high temperature range of 600° C. or more, and this report does not mention any conductivity at room temperature just as the previous report. Further, this report describes neither transparency nor applicability to transparent conductive films.
Omata et al. reported that ZnGa2O4 exhibits 30 S/cm of conductivity and has an absorption edge at 250 nm (The Ceramic Society of Japan, preliminary reports of Seminar '93, p 585). Omata et al. made a detailed report in “Applied Physics Letters, Vol. 64, p 1077, 1994”. According to this report, a sample was a sintered body prepared by mixing ZnO and Ga2O3 powders, pre-burning the mixed power at 1000° C. for 24 hours, pressingly shaping the pre-heated power into a disc-shaped body, and heating the disc-shaped body at 1300° C. for 48 hours. This sample exhibited no conductive property.
The sample was then subjected to an annealing treatment at 700° C. under hydrogen atmosphere. The resulting sample has 30 S/cm of electric conductivity. An absorption edge wavelength was determined from a diffuse reflectance spectrum of the sintered-body sample. As above, this report shows neither conductive property verified in a thin-film sample nor transparency verified by measuring a transmittance spectrum.
Kawazoe, coauthor of this article, studied more detail and made a report in “Journal of American Ceramic Society, Vol. 81, 1998, p 180”. According to this report, a ZnGa2O4 thin-film was prepared through a sputtering method. The resulting ZnGa2O4 thin-film was insulative, and no film having a conductive property was obtained. A sample formed as a single crystal was also insulative, and it had a conductive property only after a heat treatment at 600° C. under hydrogen atmosphere. However, the conductive property was verified only at a surface layer having a depth of about 50 μm from the surface of the single crystal, and the inside of the single crystal was still insulative.
In an analysis using a transmission microscope, the above surface layer had a modified crystal structure, specifically a trigonal crystal type ordered halite structure represented by 2(Zn0.5GaO2) modified from its original spinel type ZnGa2O4. This would be caused by vaporization of oxygen together with Zn in the heat treatment under hydrogen atmosphere. It is presumed that the crystal structure in the surface of the sintered-body sample prepared by Omata et al. must be changed into a trigonal crystal type ordered halite structure because the sample was also subjected to the heat treatment under hydrogen atmosphere.
The inventers have precedently proposed a material represented by a general formula Zn (Ga(1-x)Alx)2O4 or a solid solution of a spinel type crystal structure, which exhibits a conductive property and a transparency to a light having a wavelength of 250 nm or less. However, there is no prior art disclosing a ZnGa2O4 thin-film having transparent conductivity.
(Problem to be Solved by the Invention)
In late years, a light-emitting material and device having a function of emitting blue light or ultraviolet light and a solar cell for converting sunlight into electric power has been increasingly used in society. In these electronic devices, a transparent electrode is one of essential elements, and the Indium Tin Oxide (ITO) and Antimony doped Tin Oxide (ATO) are used in light-emitting devices and solar cells, respectively. However, neither the ITO nor the ATO allows sufficient transmission of a blue light having a wavelength of about 400 nm and an ultraviolet light having a wavelength shorter than the blue light. Thus, if the thickness of the transparent electrode is increased, the light-emitting devices will have a significantly reduced light-emitting efficiency. In addition, the solar cells cannot pick up any ultraviolet light from sunlight. Further, no antistatic film allowing transmission of ultraviolet light has been developed.
A halftone layer has been made of a substance having no conductive property. A phase-shift mask is prepared by covering a part of the surface of the mask with the halftone layer, which can fabricate a resist pattern with a resolution of a half of an irradiated light wavelength. The light transmittance of the halftone layer to the irradiated light is reduced down to about 4 to 20%, and is operable to shift the phase of the irradiated light by half-wavelength. Thus, the phase of the light after passing through the halftone layer is shifted by half-wavelength as compared to the light before passing therethrough. Interference of the two beams provides significantly enhanced resolution of the pattern.
The halftone layer is prepared by uniformly forming a film on the surface of a glass substrate and then patterning through an electron-beam lithography. Since the conventional halftone layer had no conductive property, electrostatic charge due to irradiated electrons during the electron-beam lithography has been prevented by applying a conductive organic material onto the surface of the halftone layer or by incorporating a conductive material such as SnO2 in the halftone layer. If an ultraviolet-transparent conductive film is used as a material for forming the half tone layer, the process of applying the organic material can be omitted, and the structure of the halftone layer can be simplified.
A concept “Lab-on-a-Chip” is one of research tasks actively addressed recently. In this concept, a micro cell formed in the surface of a Si substrate or a SiO2 glass substrate is used as a reaction vessel or analytical vessel, i.e. a micro experimental system. Various related researches are driving forward, particularly for the purpose of synthesization and/or analysis of organic molecules such as DNAs, protein molecules, medical agents and photoelectron functional organic molecules.
Generally, these organic molecules are activated by an ultraviolet light having a wavelength of about 300 nm. Thus, it is important to irradiate the organic molecules with an ultraviolet light while applying an electric field thereto or to detect a resulting ultraviolet emission. However, there has not been developed any ultraviolet-transparent conductive film usable for this purpose. If such a suitable ultraviolet-transparent conductive film is achieved, it can be effectively used as a transparent electrode in devices such as the Lab-on-a-Chip for synthesizing and/or analyzing DNAs, protein molecules or other organic molecules.